The takeover, which took last month, followed a period of negotiation between the two accountancy firms.
David Cook, director at TTR Barnes, said: "We are delighted to continue our client and business growth with our acquisition of TaxAssist. This will strengthen our position as the leading independent accountancy firm in Sunderland and our regional growth strategy.
"Our focus is on continuing our service delivery standards to our existing clients and building relationships with our new clients."
The company is ensuring a smooth changeover for new clients through the retention of key Tax Assist staff during and after the transitionary period. Peter Stoddart, Sunderland franchise operator of TaxAssist, will be maintaining his client responsibilities during the phased handover period.
TTR Barnes offers a range of both broad and specialist accounting, taxation and payroll services with its team of over 40 accountants and support staff.